Table 2.

Summary of the shortfalls of each power source.

Power source	Shortfall
Batteries	Low recharge cycles; low energy density; low flight time in comparison; recharge period significantly longer than others; dangerous to the environment and/or operators; limited flight time thus limited applications [58, 59, 60].
Hydrogen FCs	Larger size; limited by availability of hydrogen gas and gas tank size; quite expensive; lower energy efficiency compared to batteries due to complex power management requirements [61]; hydrogen extraction process increases refuel time [62];
Combustion engine	Heavier; larger size; noisy; complex maintenance;
Solar power	Large surface required for solar panels; requires sunlight; much heavier than others; significantly larger cost than others; maximum power point tracking (MPPT) algorithm is required [63].